A multi-mode electrically variable transmission is an advantageous new transmission design that has the ability to reduce engine and electric motor losses at low as well as high vehicle speeds. However, depending on the implementation of the mode-changing mechanism, a multi-mode electrically variable transmission (“multi-mode EVT”) has potential disadvantages. For example, a multi-mode EVT may experience increased electric motor losses at higher vehicle speeds (“mechanical point chasing”) and higher transmission spin losses due to clutch drag and planetary gear set friction. Further, the gearing range within some multi-mode EVTs may limit the electric vehicle (“EV”) drive capabilities.
A simplified illustration of mechanical point chasing in FIG. 1. shows the rotations per minute (“RPM”) of a first electric motor A, second electric motor B, and engine of a typical prior art single-mode electrically variable transmission plotted against the output rotations per minute of the final drive. As can be seen in FIG. 1, the engine RPM (“ENGINE”) remain constant as the final drive RPM increases until the final drive RPM reaches N1. Meanwhile, the RPM of electric motor B (“MOT B”) increases proportionally with the final drive RPM. In contrast to electric motor B, the RPM of electric motor A (“MOT A”) decreases proportionally with the final drive RPM and is equal to zero at a final drive RPM of N1. Once the RPM of electric motor A reaches small negative values (0 rpm used in this simplified example), any further decrease in RPM of electric motor A results in correspondingly increased system losses. Thus, typical single-mode EVT transmissions will not allow electric motor A to operate at (very) negative RPM. The rotation speed of the engine must account for the lack of a further decline (into negative) of RPM of electric motor A above final drive RPM of N1. Thus, at final drive RPM above N1, the engine RPM must increase proportionally with the increase in the final drive RPM. An increase in engine RPM likely results in the engine operating out of its optimum fuel efficiency or power range, or both. Thus, it becomes necessary to design the gearing of the EVT transmission to compromise between adequate highway (i.e., high-speed) and city (i.e., low-speed) performance and efficiency. Thus, a desirable EVT keeps the engine operating within its efficiency and/or power range while still providing satisfactory city and highway performance, while also being compactly implementable with a low loss mode-change mechanism.
In addition, because the gearing of the EVT must be designed with the compromise between city and highway driving in mind, the gearing of the EVT will often be higher than desirable for city driving in order to achieve adequate vehicle speeds during highway driving. Therefore, the electric motors of the EVT often must provide higher torque levels than necessary for city driving to overcome the taller final drive ratio of the EVT. Higher torque electric motors are typically larger and more expensive than lower torque models. At the same time, because of the compromise between city and highway driving, the final drive ratio is not optimum for purely electric operation. When operating purely under battery power without the propulsive force from the engine, the maximum final drive speed is limited by the component speeds of the planetary gear set within the EVT. Under purely electric power, EVTs typically have a limited top speed resulting from the city and highway gearing compromise. Thus, a desirable EVT allows for a shorter final drive ratio so that smaller and less powerful electric motors may be used as well as a taller final drive ratio that allows for adequate vehicle speeds during electric vehicle operation.
An EVT generally has limited reverse gear operation and relies solely upon electric motors to provide reverse propulsion. This is problematic during situations in which electric battery power or electric motor torque may be limited such as in extremely hot or cold climates. If electric power fails or provides inadequate propulsive force, the vehicle is simply unable to move in reverse. Thus, a desirable EVT harnesses the propulsive force of the engine for reverse gear operation or uses both electric motors of the EVT for reverse gear operation if battery power is adequate.
It is, therefore, desirable to provide an EVT that keeps the engine operating within its efficiency and/or power range while also providing for satisfactory city and highway performance. It is, therefore, also desirable to provide an EVT that allows for lower torque motors to be used within the transmission while still achieving satisfactory city performance and adequate speeds during EV operation. It is also desirable to provide an EVT in which the engine may provide propulsive force for reverse gear operation.